The present invention relates to a secondary cell residual capacity calculation method and a battery pack. More specifically, the present invention relates to a residual capacity calculation method in a battery pack incorporating a microcomputer, and the battery pack.
At present, there is provided an electronic apparatus incorporating a secondary cell such as a lithium ion secondary cell, for instance, as a portable electronic apparatus power source. In the electronic apparatus incorporating the lithium ion secondary cell, a full charge is detected to prevent an overcharge state of the lithium ion secondary cell, and a final discharge voltage is detected from a terminal voltage of the lithium ion secondary cell to prevent an over-discharge state, likewise. For instance, the electronic apparatus is arranged to provide control so as to bring a system to a stop when the final discharge voltage is detected.
As charge-and-discharge control of the electronic apparatus as described above, there is provided, for instance, a residual capacity calculation method (which will be hereinafter referred to as a voltage method) utilizing the terminal voltage of the lithium ion secondary cell.
Specifically, there is provided a technology of expressing a terminal voltage-to-residual capacity relation of a charging-type cell at the discharging time, of calculating, with reference to a map, a capacity (zero capacity) that allows discharging to be stopped, and of correcting a cumulative residual capacity using the above zero capacity to make a correction of effects of a cumulative error due to a current integration, a capacity change with load current of the charging-type cell and a capacity change with deterioration, and the like thereby permitting a residual capacity to be detected with high accuracy, also permitting an error in the zero capacity to be minimized by using a three-dimensional map created with a relation additionally including a discharge current, and further permitting a detection accuracy of the residual capacity to be further increased by using a four-dimensional map further including a deterioration characteristic. See Japanese Patent Application Publication No. 2001-281306.
However, the terminal voltage of the secondary cell largely undergoes fluctuations with parameter such as current flowing to a connected load, temperature, deterioration, and the like, so that more accurate control using the voltage method needs to provide, as a table, for instance, a reference value for each parameter.
An arrangement of the table for each of all the parameters requires numerous tables, which lead to a waste of a ROM (Read Only Memory) incorporated in a microcomputer, resulting in a problem of being difficult to increase the number of tables randomly.
Further, data contained in the tables are calculated from results having been evaluated using practical parameters, so that a high accuracy is maintainable under a supposed environment, whereas a problem of being liable to cause an unexpected large error only by a slight deviation from the practical parameters arises.
Furthermore, decreasing the number of tables may reduce a storage capacity, but leads to a reduction in number of parameters specified as references, so that a value specified as an inter-table (inter-condition) value requires a computational expression for interpolation of the parameters. The interpolation in the presence of a need to perform inter-parameter interpolation involves a problem of being extremely difficult to suppress an error caused by the interpolation performed in consideration of parameter interactions.
A current integration method generally available as the residual capacity calculation method in the lithium ion secondary cell is of a system adaptable to meet a lithium ion secondary cell characteristic in which a charging electricity quantity Q and a discharging electricity quantity Q are both equal. This method enables a residual capacity calculation with high accuracy in an initial stage of the secondary cell after fabrication thereof.
However, effects of a measurement error, a self-discharge in a long-term preservation condition and a cell deterioration etc. lead to a need for cancellation of a cumulative electricity quantity error, that is, learning, with the lapse of a long time since the fabrication or with the repetition of charging and discharging over several hundred times. At this time, while the terminal voltage is measured as correct reference data required to allow the learning to be performed, the learning in the case of the lithium ion secondary cell takes place generally in a last discharge stage that is subject to a large discharge voltage change.
However, great fluctuations of values between a component having been obtained by a calculation using the current integration until now and a component obtained by an error cancellation in such a manner as to perform the learning using a result of voltage measurement occur frequently. Particularly, in the case of cells, such as a cell adapted to a camera for business use, in which an accuracy of the calculation on a residual discharge time in the last discharge stage is considered to be the most important point, an execution of the learning at the last discharge stage involves a problem of being incapable of highly accurate calculation on the residual discharge time in the last discharge stage, as a matter of course.
Further, when the highly accurate calculation on the residual capacity is required over a long period of time, the learning should be performed without incurring any erroneous learning. However, it is very difficult to allow the erroneous learning to be prevented, when learning data is set on the basis of a voltage. This may be attributed to the fact that the terminal voltage of the lithium ion secondary cell undergoes voltage fluctuations with temperature, and also causes large voltage fluctuations by a deterioration condition and by abrupt load fluctuations and the like. It is impossible to allow the microcomputer to obtain these parameters securely.